Split the JSON responses into one key per section

Previously, the AJAX update for the dashboard was returning a big blob
of JSON with one key.

This commit splits it up to return:

- one key for each section of the page
- each containing a smaller chunk of HTML rendered from a partial

The jobs page was already working this way (pretty much) but just needed
a little tweaking to get it the same.

app/templates/partials/jobs/notifications.html

@@ -1,17 +1,6 @@
 {% from "components/table.html" import list_table, field, right_aligned_field_heading, date_field, row_heading %}
 
-<div
-  {% if notifications %}
-    class='dashboard-table'
-  {% endif %}
-  {% if not finished %}
-    data-module="update-content"
-    data-resource="{{url_for(".view_job_updates", service_id=current_service.id, job_id=job.id, status=request.args.get('status'), help=request.args.get('help'))}}"
-    data-key="notifications"
-    aria-live="polite"
-  {% endif %}
->
-
+<div class="dashboard-table">
   {% if notifications %}
     <p class="bottom-gutter">
       <a href="{{ url_for('.view_job_csv', service_id=current_service.id, job_id=job.id, status=status) }}" download="download" class="heading-small">Download as a CSV file</a>
@@ -45,5 +34,4 @@
       {{ item.status|format_notification_status(item.template.template_type) }}
     {% endcall %}
   {% endcall %}
-
 </div>